1. Field of the Invention
The present invention generally relates to nuclear reactors and, in particular, to devices for moving control rods with respect to reactor cores.
2. Description of the Prior Art
One of the most serious accidents that can occur to a nuclear power plant is a loss of coolant followed by the failure of the control system to accomplish a rapid shutdown of the reactor. A loss of coolant flow can occur from either the rupture of piping or the stoppage of one or more of the coolant circulating pumps. This type of accident is especially serious because the heat generated in the reactor cannot be carried off. If the reactor continues to generate heat, then tremendous pressures are built up in the coolant system. In addition, this heat generation, if it is not terminated by a scram, could melt down a majority of the core of the reactor.
In the reactors using liquid sodium for primary coolant, there is a special problem caused by a partial or total loss of sodium flow if a reactor scram does not follow promptly. In the present design of liquid metal fast breeder reactors there is a gain in reactivity called a positive sodium void coefficient that occurs when sodium flow is interrupted. The sodium temperature may increase to its boiling point, whereupon "voids" of sodium vapor are formed, resulting in increased reactivity, power, more boiling, and the possibility of serious consequences. This gain in reactivity occurs because although the neutron absorption effect of sodium is small, it is not zero. Any loss of of sodium from the core causes a shift in the neutron absorption spectrum and increases the number of neutrons. This shift, in turn, increases the probability of neutron capture by the fissionable atoms in the fuel.
Although the reactor control systems and scram circuits in use today are quite reliable, there is a continuing effort in the nuclear reactor industry to make reactor control systems absolutely foolproof. No one in this industry is entirely satisfied with the present state of the art. There is a continuous emphasis on making reactor shutdown systems as reliable as possible. These efforts include minimizing the number of moving mechanical parts and designing the components to be independent of both electrical failures and sensor failures.